1. Field of the Invention
The present invention relates to particles for electrophoretic display, a liquid dispersion of the particles for electrophoretic display, and an electrophoretic displaying device utilizing migration of the electrophoretic particles.
2. Related Background Art
In recent years, with development of information instruments, needs are increasing for thin low-power-consuming displaying device as well as printing mediums, and research and development are being conducted actively for the displaying device to meet the needs.
Particularly, liquid crystal displaying devices, which utilizes change of optical properties of the liquid crystal by controlling electrically the arrangement of the liquid crystal molecules, are being actively developed and are commercialized. Colorization is also being developed for more natural display. In the colorization, color images are displayed by using color filters and liquid crystal shutters.
However, the liquid displaying devices have disadvantages that letters and characters in the image are not readily identifiable depending on the viewing angle and reflected light, and visual tension can be caused by flickering and low luminance of the light source. The disadvantages have not been cancelled effectively. Therefore, researches are being conducted for the displaying device which does not cause visual tension.
Reflection type displaying devices are promising in view of lower power consumption and less visual tension. One example is the electrophoretic displaying device disclosed in U.S. Pat. No. 3,612,758 by Harold D. Lees, et al.
Another example of the electrophoretic displaying device is disclosed in Japanese Patent Application Laid-Open No. 9-185087. This apparatus is explained below.
FIGS. 5A and 5B illustrate an example of the structure of the aforementioned electrophoretic displaying device. The displaying device is constituted of a pair of base plates 13a, 13b placed at a prescribed interval, dispersing medium 12 filled between base plates 13a, 13b, electrophoretic particles 11 dispersed in dispersing medium 12, and first electrode 14a and second electrode 14b placed along base plate 13b. In this apparatus, electrophoretic particles 11 are electrified positively or negatively, and are allowed to migrate by electrophoresis onto electrode 14a or 14b in accordance with the polarity of the voltage applied to electrodes 14a, 14b. Electrophoretic particles 11 and the areas where the electrodes 14a, 14b are placed are colored, thereby an image is displayed as the whole displaying device. In the drawings, the numeral 15 indicates partition walls, and the numeral 16 indicates an insulation layer.
The electrophoretic displaying device displays an image by masking the electrode with electrified electrophoretic particles. The contrast of the display can be lowered by insufficiency of the masking power of the electrophoretic particles or by the color density. Therefore, further improvement is necessary for higher display quality.
Japanese Patent Application Laid-Open No. 55-103527 discloses an electrophoretic displaying device employing a liquid electrophoretic dispersion containing white particles formed by coating titanium oxide pigment with polyethylene as a specific example of the electrophoretic particles and the electrophoretic display liquid.
U.S. Pat. No. 6,117,368 discloses black particles prepared by dyeing of white polymer particles as the electrophoretic particles.
Japanese Patent Application Laid-Open No. 2000-206574 discloses an electrophoretic displaying liquid constituted of a dyed dispersing medium and particulate pigments having different particle diameters.
On the other hand, Japanese Patent Application Laid-Open No. 08-295837 discloses pigment-containing fine polymer particles for ink-jet as a durable particulate containing a pigment.
U.S. Pat. No. 5,914,805 discloses a displaying device employing a mixture of two kinds of twist balls having different particle diameters. This apparatus improves the light reflectivity and electrode masking effect by allowing small-sized twist balls to fill the void around large-sized twist balls.
The present invention intends to provide particles for electrophoretic display (the particles hereinafter being referred to as “electrophoretic displaying particles”) having a high color density and a high masking power, an electrophoretic displaying dispersion liquid containing the electrophoretic displaying particles and a dispersing medium therefor, and an electrophoretic displaying device employing the electrophoretic display dispersion liquid.